Temperature detection cells are generally used in integrated circuits as alarms to indicate an abnormal heating of these integrated circuits. The use of such cells makes it possible to stop the operation, and therefore, the heating of the integrated circuit before it is damaged as a result of the heat.
The principle of operation of such a cell is summarized in FIG. 1. The cell has a circuit producing a voltage VR that increases with the temperature T, and is compared with a voltage VBEON that diminishes with the temperature T. At ambient temperature T0, VBEON is higher than VR. When the temperature in the vicinity of the cell increases, the two voltages approach each other and the cell produces an alarm signal VOUT when the voltage VBEON becomes lower than the voltage VR (detection threshold TD).
An example of such a cell is shown in FIG. 2. It comprises a current source 11, three N-type transistors MN1, MN2, MN3, two P-type transistors MP1, MP2, one bipolar transistor Q1, one resistor R and two inverters 12, 13.
A power supply potential VPLUS is applied to one of the terminals of the source 11 whose other terminal is connected to the drain of the transistor MN1. The potential VPLUS is also applied to the source of the transistor MP1 whose drain is connected to one of the terminals of the resistor R having its other terminal connected to the drain of MN2. The potential VPLUS is also applied to the source of MP2 whose drain is connected to the drain of MN3. The gates of MP1 and MP2 are connected together to the drain of MP1.
The emitter of Q1 is connected to the common point of the resistor R and of the transistor MP1. The base of Q1 is connected to the common point of the resistor R and of the transistor MN2. The gates of the transistors MN1, MN2, MN3 are connected together to the common point of MN1 and of the current source 11. The two inverters 12, 13 are series-connected. The input of the inverter 12 is connected to the common point of MP2 and MN3, and the output of the inverter 13 forms the output VOUT of the detection cell. The inverters 12, 13 simply have the effect of converting the analog potential VComp present at the common point of the transistors MP2, MN3 into a digital signal VOUT which is inactive if the temperature is below the detection threshold TD of the cells, and is active if not. Finally, a ground potential VMINUS is applied to the source of the transistors MN1, MN2, MN3 and the collector of the transistor Q1.
The transistors MN1, MN2 are identical and form a current mirror. The current I produced by the source 11 crosses the transistor MN1 which copies it into the transistor MN2. The current I thus flows in the resistor R. The transistors MP1, MP2 are identical and also form a current mirror. The current flowing in the transistor MP, which is equal to the sum of the current flowing in the resistor R and the current flowing in the emitter of the transistor Q1, is copied into the transistor MP2. Finally, the transistors MN1, MN3 also form a current mirror. However, the transistor MN3 is chosen such that the current copied out into the transistor MN3, proportional (according to the principle of the current mirror) to the current I flowing in the transistor MN1, is also slightly greater than the current flowing in MP2. The transistor Q1 has a base-emitter voltage VBEON that decreases with the temperature (FIG. 1). This is a well-known characteristic of bipolar transistors.
The current source 11 is formed according to a known scheme using bipolar transistors. The current I produced by the source 11 is proportional to the difference (referenced ΔVBE) between the base-emitter voltages of two bipolar transistors, and the current I increases with the temperature. This is due to the well-known temperature characteristics of the bipolar transistors. I=ΔVBE/RI is written, with RI being a constant. The current source 11, the transistors MN1, MN2 and the resistor R together form the circuit that produces a voltage VR increasing with the temperature (FIG. 1). ΔVBE, I and VR follow the same progress as a function of the temperature.
In normal operation, the current I given by the source 11 is low, so that the voltage VR at the terminals of R (equal to R*I) is low and the transistor Q1 is off. The current in the emitter of Q1 is therefore zero and the current flowing in the transistors MP1, MP2 is equal to I. Finally, the current I flowing in MN1 is copied out into MN3. Since the current flowing in MN3 is greater (MN3 has been chosen accordingly) than the current I flowing in the transistor MP2, the common point of the transistors MP2, MN3 is brought to the potential VMINUS and the output VOUT is equal to a logic zero. When the temperature rises, the voltage VR and the current I rise, while the potential VBEON diminishes.
When the temperature crosses the detection threshold TD of the cell, the current I produced by the source 11 is great. It is such that the voltage VR is higher than the conduction threshold VBEON of the transistor Q1 which comes on. A current flows in the transistor Q1 and is added to the current I in the transistor MP2. The current I added to the current flowing in the emitter of Q1 is copied into MP2. Since the current flowing in MP2 is greater than the current flowing in MN3, the current MP2 draws the common point of the transistors MP2, MN3 to the potential VPLUS and the output VOUT becomes equal to a logic one. This indicates that the temperature has reached the detection threshold TD. The detection threshold TD is reached when the temperature is such that the voltage VR becomes equal to the emitter-base voltage at which Q1 comes on and conducts a current.
One problem with present-day detection cells is the variation of the detection threshold from one cell to another. Despite all the care taken in designing a series of cells, the values of the resistors, the voltages ΔVBE, VBEON of the bipolar transistors vary by a few percentage points from one cell to another in the same production line. Due to these variations in parameters, the voltages produced by the bipolar transistors and the currents produced by the bipolar transistors or copied by the current mirrors also vary from one cell to another. These errors generally accumulate and finally have a considerable influence on the value of the detection threshold of the cell.
It is noted that for a series of cells sized to have a given theoretical detection threshold TD, it is possible to have cells whose real detection threshold diverges by 20% to 30% from the desired value, which is unacceptable for certain applications. The only way to guarantee the value of the detection threshold of the cell is to measure it.
At present, the only known test method (the measurement of the temperature threshold TD) for a cell is a test in real conditions in which the temperature in the neighborhood of the cell is gradually increased until it reaches the detection threshold. For obvious reasons of cost and time, such a test is performed on a very limited sample of cells coming from a same production batch of several tens of thousands of cells. This does not guarantee that the detection threshold of the cell is taken individually.